<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        .box1 {
            margin: 0;
            padding: 5px;
            height: 200px;
            background-color: #ddd;
            position: relative;
        }
        button {
            margin: 5px;
        }
        .box2 {
            width: 100px;
            height: 100px;
            background-color: red;
            position: absolute;
            left: 0;
        }
    </style>
</head>
<body>
<div class="box1">
    <button>运动到200</button>
    <button>运动到400</button>
    <div class="box2"></div>
</div>


<script>

    var btnArr = document.getElementsByTagName("button");
    var box2 = document.getElementsByClassName("box2")[0];

    var timer = null;

    //绑定事件
    btnArr[0].onclick = function () {
        animate(200);
    }

    btnArr[1].onclick = function () {
        animate(400);
    }


    function animate(target){
        //要用定时器，先清除定时器
        clearInterval(timer);
        //我们要求盒子既能向前又能向后，那么我们的步长就得有正有负
        //目标值如果大于当前值取正，目标值如果小于当前值取负
        var speed = target>box2.offsetLeft?10:-10;
        timer = setInterval(function () {
            //在执行之前就获取当前值和目标值之差
            var val = target - box2.offsetLeft;
            box2.style.left = box2.offsetLeft + speed + "px";
            //目标值和当前值只差如果小于步长，那么就不能在前进了
            //因为步长有正有负，所有转换成绝对值来比较
            if(Math.abs(val)<Math.abs(speed)){
                box2.style.left = target + "px";
                clearInterval(timer);
            }
        },30)
    }


</script>
</body>
</html>